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Novel successive phase transitions and phase diagrams of Rb4Mn(MoO4)3:  A model compound in quasi-two-dimensional antiferromagnet

Nakatsuji, Sakakibara, Tokunaga, and Kawashima Groups

Geometrically frustrated magnetism has been a subject of active research in condensed matter physics. Generally in physics, quantitative comparison between experiment and theory is crucial to make a firm progress of our understanding. In the field of frustrated magnetism, however, such fortuitous cases are still scarce where full or semi quantitative agreement between experiment and theory have been found, except a few examples such as spin ice [1] and the orthogonal dimer SrCu2(BO4)2 [2].

Fig.1. Crystal structure of Rb4Mn(MoO4)3 with equilateral triangular lattices of Mn2+ and MoO4 tetrahedra.

Fig.2. H-T phase diagrams of Rb4Mn(MoO4)3 for (a) μ0Hc and (b) μ0Hab constructed by using various experimental techniques, and by Monte Carlo simulations.

Two-dimensional (2D) triangular antiferromagnets have been extensively studied, because of rich frustrated magnetism expected on the simple 2D Bravais lattice. Theoretically, it has gained a consensus that the ground state for the nearest-neighbor antiferromagnetic (AF) Heisenberg model has the 120◦ spin order. In this case, the concept of vector spin chirality, the handedness of the way the spins are rotated in a 120°order for a given triangle, may tron diffraction measurements confirmed that the structure is consistent with the X-ray results and stable down to 1.5 K. Thus, Rb4Mn(MoO4)3 has become essential and lead to exotic phenomena such as phase transitions with a new universality class and multiferroic phenomena. However, neither detailed study of the phase diagram under external field nor quantitative comparison between experiment and theory has been made so far, because of a relatively large scale of the antiferromagnetic coupling J and/or lattice deformation due to magnetostriction.

Recently, we have succeeded in growing single crystals of the quasi-2D Heisenberg triangular antiferromagnet Rb4Mn(MoO4)3 and investigated its magnetic properties [3]. Powder neu

the equilateral triangular lattice formed by Mn2+ with S = 5/2 (Figure 1). We have made a comprehensive study on the crystal/spin structures and thermodynamic properties of the quasi-2D Heisenberg triangular antiferromagnet Rb4Mn(MoO4)3 [3]. This material exhibits the successive transitions and 1/3 magnetization plateau phase under field, reflecting its easy-axis anisotropy. As a rare case in geometrically frustrated magnets, quantitative agreement between experiment and theory has been found for the phase diagrams (Figure 2) and magnetic properties, establishing the system as a model 2D Heisenberg triangular antiferromagnet characterized by the nearest neighbor Heisenberg Hamiltonian.

Our detailed quantitative agreement between experiment and theory for this geometrically frustrated magnet has established Rb4Mn(MoO4)3 as a model system to explore many unusual aspects of 2D frustrated magnetism, such as critical dynamics associated with vector chirality, multiferroic noncolliner magnetism, and instability of conventional magnon excitations.


References
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  • H. Kageyama, K. Yoshimura, R. Stern, N. V. Mushnikov, K. Onizuka, M. Kato, K. Kosuge, C. P. Slichter, T. Goto, and Y. Ueda, Physical Review Letters 82, 3168 (1999).
  • R. Ishii, S. Tanaka, K.Onuma, Y. Nambu, M. Tokunaga, T. Sakakibara, N. Kawashima, Y. Maeno, C. Broholm, D. P. Gautreaux, J. Y. Chan, and S. Nakatsuji, Europhysics Letters 94, 17001 (2011).
Authors
  • bJohns Hopkins University
    cLouisiana State University
  • R. Ishii, S. Tanaka, K. Onumaa, Y. Nambu, M. Tokunaga, T. Sakakibara, N. Kawashima, Y. Maenoa, C. Broholmb, D. P. Gautreauxc, J. Y. Chanc, and S. Nakatsuji
    aKyoto University